1. Field of the Invention
The present invention relates to an apparatus for measuring radiation characteristic of a laser diode. More particularly, the invention concerns a radiation characteristic measuring apparatus which is protected from the influence of change in a DC (direct current) voltage upon measurement of differentiated characteristic of a laser diode by applying thereto an AC (alternating current) voltage in superposition to a step-like DC (direct current) voltage.
2. Description of the Prior Art
FIG. 2 of the accompanying drawings shows a radiant-power or photo-output versus excitation- or drive-current characteristic of a laser diode. This characteristic curve provides a criterion for deciding whether a laser diode in concern is to be satisfactory or not in respect to the characteristic. More specifically, when the characteristic curve as measured is of a form fluctuating unnaturally, it may be decided that the laser diode in concern has poor characteristic. Accordingly, decision as to whether a laser diode is to be satisfactory or not in respect to the characteristic may be realized by differentiating a signal representative of the characteristic curve illustrated in FIG. 2. In a practical application, the differentiation may be carried out by applying a stepwise increasing DC voltage to the laser diode and measuring the radiant power output thereof. When the signal derived through the differentiation represents a uniformly increasing curve, it may be decided that the diode as tested is of a good quality, while fluctuation present in the curve resulting from the differentiation will means that the laser diode in concern is not to be satisfactory in respect to the characteristic in concern. It is however noticed that a problem is often encountered in making the decision mentioned above because the data obtained through differentiation is often of so small magnitude that discrimination is rendered very difficult. In an attempt to avoid such difficulty, it has been proposed to apply an AC voltage to the laser diode under test in super-position to the DC voltage, wherein the differential measurement is performed by amplifying the AC voltage from the laser diode. In that case, however, when a step-like DC voltage is employed, data as obtained undergoes a change every time the DC voltage changes stepwise, making it difficult to discriminate whether the detected data increases uniformly or suffers fluctuation.